how in the world do you guys have so many characters?

I've been playing since mid-December, and I have 20 characters in my roster at the moment, ranging from 60s (Wolverine) all the way down to about 15 (Single Cover 3*** for example)

I didn't spend money to get anything, but it's important to note that I was on break during December and part of January so I had a large amount of time. That's ultimately what it comes down to. If you don't want to put money into it, you'll need to place time. Whether that's trying to blitz through content fanatically in a short period of consistently playing over a longer period of time, that's up to your schedule.

The general idea is to play through the prologue to pick up as much as you can. Starting from the beginning, you'll want to max out the optional rewards (usually up to 4) from each mission.
Then there's events (PvE ones). When you start out, it may be hard to clear the content, but there's usually a few easy ones at the start to help you pick up some rewards both after missions and when the event ends.
Then there's PvP. While placing high is unlikely when your roster is small, you could alternatively try to grind your way up to progression rewards, some of which can be very vital. There's generally recruit cover at very low progression levels so if you can squeak out the 50pts or whatever to grab your token for a roll of the die, you should try.

I'd focus on them in about that order. I don't really know what to specifically recommend as far as roster because there's major changes coming to the previous staples (Wolverine and Thor) so the future is unclear.

In closing, or tl;dr: Keep moving through the prologue, pick up as many rewards as you can and try to get your feet wet in the PvE events and if you think you can, choose your battles wisely in PvP.

I've been playing for what... 5 weeks now? (Huh, feels more like just a half that time...). And yes, I've been feeding some money to the devs during that. I've bought HP with my credit card to get roster slots, Heroic tokens (at the hindsight I really don't recommend it: due time you can get those covers by other means fairly easy), health packs and shields for 70-80$ worth. I don't regret it THAT much but now when the novelty of MPQ has worn off I'm quite a lot more conservative player when it comes to spending real money. No more Visa-HP unless I really need those health packs for some "important" PvP push. I don't want to invest any more money until I see where exactly this game is headed to.

As previously said you really need to have either lots of time or some money to advance quicker. But if you don't count those competitive PvP matches, why hurry? The beginning can be very overwhelming but I'm perfect example of those who should have kept their head cool and just played for a bit longer with low-level chars and slowly leveled my team. Would have been a lot cheaper. But noooo... I HAD to have those Wolverine covers quicker.